Bismillaah Al-Hamdulillaah wa salatu wa salaamu 'ala rasulullaah
Amma-ba'd
From Shaikh Al-'Uthaimeen's highly beneficial explanation of the Du'aa of Qunoot ------> http://www.tarbiyyahpublishingonline...a%20Qunoot.pdf
it becomes clear there is scholarly ikhtilaaf regarding the issue of adding additional du'aa to the legislated Qunoot.
Shaikh al-'Uthaimeen (rahimahullaah) was asked (on page 24):
Q: Is it permissible to add additional words to this Du'aa, which the Prophet (salallaahu 'alayhi wa sallam) taught al-Hasan Ibn 'Alee?
A: There is no problem should a person wish to increase this Du'aa of Qunoot during the Witr prayer with additional words. If he were alone (in prayer), then he may also make Du'aa with whatever he likes. However, it is preferable for a person to choose comprehensive and general supplications as the Prophet (salallaahu 'alayhi wa sallam) used to supplicate with comprehensive, general supplications, and other times would supplicate with ones that are more specific. It is also incumbent upon the Imaam not to prolong it upon the people so not as to put a burden or hardship upon them.16
16- Taken from “Majmoo’ Fataawaa wa Rasaail” (14/138).